This invention relates to an optical system having a diffractive optical element. More particularly, the invention concerns an optical system suitably usable in a semiconductor manufacturing apparatus, for example, for printing, by projection exposure, a device pattern formed on a reticle or a mask (hereinafter, mask) on different locations on a wafer in accordance with a step-and-repeat method or a step-and-scan method, to produce various devices having a pattern of submicron or quarter-micron size or smaller, such as ICs, LSIs, CCDs or liquid crystal panels, for example.
Most optical systems, including projection optical systems, of semiconductor manufacturing apparatuses are constituted by dioptric systems only. Recently, however, many proposals have been made to an optical system using a diffractive optical element (DOE). Examples of such diffractive optical elements are a phase type diffractive optical element or an amplitude type diffractive optical element, known as a Fresnel zone plate. In the amplitude type, a portion of light is blocked by the optical element and, therefore, it is undesirable with respect to the efficiency of light utilization. In the phase type diffractive optical element, on the other hand, it is known that, if it is assuredly manufactured to provide an idealistic phase change, a diffraction efficiency of 100% is attainable. Particularly, those called a surface relief type are used in many cases in an ordinary optical system. In this type, a structure is defined in the depth direction of the element substrate, by which a phase change corresponding to the position on the element surface is applied to the light passing therethrough. The depth which is normally required is of a wavelength order, and the thickness of the element can be made small. Further, various phase changes can be accomplished by changing the position of the structure. Thus, for ordinary dioptric systems, an effect such as attainable by forming an aspherical surface can be attained widely. The function for describing phase changes applied to light in accordance with the position on the element surface is called a phase function.
Another feature of diffractive optical elements is that a color dispersion appears inversely to that of a dioptric system. Based on this feature, chromatic aberration produced by a dioptric system can be corrected by use of a diffractive optical element.
Due to these features, diffractive optical elements may be suitably used in a projection optical system for a semiconductor manufacturing apparatus. Conventionally, the light used with such an optical system has a wavelength of i-line (xcex=356 nm). As for such a wavelength, there are plural glass materials having a sufficient transmission factor and, for this reason, correction of chromatic aberration is attainable with a combination of dioptric optical elements. On the other hand, as regards an ultraviolet region of a currently used KrF excimer laser (xcex=248 nm) or a next-generation ArF excimer laser (xcex=139 nm), for example, glass materials having a sufficient transmission factor are only SiO2 and F2. Further, as for an F2 laser (xcex=157 nm), only CaF2 is available. Although the bandwidth of an emission spectrum of a laser light source is narrow, the imaging performance required for a projection optical system of a semiconductor manufacturing apparatus is extraordinarily high. Therefore, with an optical system constituted by dioptric systems only, there arises a problem of chromatic aberration. For this reason, a strict condition that the bandwidth must be not greater than 1 pm is additionally applied to the light source, and this necessitates a structure for narrowing the bandwidth. Further, the number of lenses required for sufficiently reducing the wavefront aberration of the optical system becomes larger, and this leads to an increase of the lens whole thickness and an increase of the surfaces where an anti-reflection film should be applied. As a result, the transmission factor of the optical system as a whole becomes lower. This means that the absorption of exposure light by the lens system as a whole increases, and it is undesirable also with respect to the aberration (exposure aberration) produced with the exposure.
Use of a diffractive optical element may be effective for problems of an increase in total lens thickness or lens surfaces or large aberration correction.
Although the advantages of diffractive optical elements themselves are known in the art, many proposals for such optical systems (e.g., Japanese Laid-Open Patent Application, Laid-Open No. 331941/1994) are made recently just after a binary optics element (BOE) is proposed. Details of such a binary optics element are discussed in G. J. Swanson, Technical Report 854, MIT Lincoln Laboratory, Aug. 14, 1989, or G. J. Swanson, Technical Report 914, MIT Lincoln Laboratory, Mar. 1, 1999, for example.
Conventionally, from the machining precision and the like, it is very difficult to directly produce an idealistic shape (blazed shape) required for a diffractive optical element, that is, a shape necessary for correctly depicting the phase function. In the case of binary optics, however, a blazed shape is not directly produced, but it is approximated by use of a step-like shape. Such a step-like shape can be produced precisely in a very fine structure, through a lithographic process and by use of a stepper as an exposure apparatus.
Now, a description will be made with reference to an idealistic lens for converging parallel light to a single point. In order that parallel light (plane wave) incident on a lens is converged to a single point, a phase function such as follows may be given:
xc3x8(r)=xe2x88x922xcfx80|(r2+f2)xc2xdxe2x88x92f|/xcexxe2x80x83xe2x80x83(1)
where f is the focal length, xcex is the wavelength of light used, and r is the distance from an arbitrary origin.
In the diffractive optical element, the fact that light has a period of 2xcfx80 with respect to the phase is used. First, the value of r=Rm with which the value of a phase function xc3x8(r) becomes equal to a multiple of 2xcfx80, by an integer, is calculated (wherein m is an integer not less than 0, and, while taking RO=0, the counting is done sequentially from the origin toward the outside), and a phase function xc3x8xe2x80x2(r) having a multiple of 2xcfx80 added is prepared so that in the period [Rm, Rm+1] the value of xc3x8(r) comes into range [0, 2xcfx80]. An optical element having its surface shaped to satisfy this phase function xc3x8xe2x80x2(r) is thus an idealistic lens based on a diffractive optical element. FIGS. 1A and 1B are schematic view of such a surface shape. The ring interval Tm may be defined as Tm=Rm+1xe2x88x92Rm. The ring interval is relatively large at the central portion (r to 0) and, depending on the difference in m, the difference in ring interval is large. On the other hand, at the peripheral portion, the ring interval is approximately regular even if the value of m differs and, therefore, it can be considered to be a regular interval grating.
Reference numeral 101 denotes the surface shape at the central portion, and it is a blazed shape which completely describes the phase function. Here, the shape 101 is a portion of a curved surface. On the other hand, reference numeral 102 denotes a blazed shape at the peripheral portion. It can be considered to be approximately a plane.
FIGS. 2A and 2B are schematic views of a surface shape, wherein an idealistic lens is manufactured as binary optics. This shape is provided by approximating the blazed shape of FIG. 1 by a step-like shape. Here, the step difference (height) of the steps may be determined so that the phase is sampled with regular intervals. Namely, if the depth in the case of the blazed shape is D and the number of approximated steps is N, each step has a height D/N. Since the step difference (height) is made constant, the width of each step is uneven in the portion 103 where a curved surface is approximated, whereas the width of each step is even at 104 where a plane is approximated.
In the binary optics, however, since the shape is based on the approximation, the diffraction efficiency does not reach 100% and unwanted diffraction light is produced. When the number of steps approximated is N and the diffraction order (design order) set to satisfy the imaging condition is 1, the diffraction efficiency xcex7Nm for the diffraction order m can be expressed as follows:                               η          m          N                =                              [                                                            sin                  ⁡                                      (                                          π                      ⁢                                              xe2x80x83                                            ⁢                                              m                        /                        N                                                              )                                                  ⁢                sin                ⁢                                  {                                      π                    ⁡                                          (                                              1                        -                        m                                            )                                                        }                                                            π                ⁢                                  xe2x80x83                                ⁢                m                ⁢                                  xe2x80x83                                ⁢                sin                ⁢                                  {                                                            π                      ⁡                                              (                                                  1                          -                          m                                                )                                                              /                    N                                    }                                                      ]                    2                                    (        2        )            
Here, the depth of the element should be optimized with respect to the wavelength xcex used. The height d of one step in this case is, assuming that the element made of a glass of a refractive index n is placed in air (refractive index 1.0), d=xcex/(nxe2x88x921.0)/N.
Generally, the bandwidth of the wavelength used in a projection optical system of a semiconductor manufacturing apparatus is about 1 pm. This is a bandwidth required because of a difficulty in correction of chromatic aberration in a lens system. Even when a diffractive optical element is combined with a lens system to enable correction of chromatic aberration in a certain range, if the light source used is a laser, the bandwidth is 1 nm at the largest. In order that the relation (2) is satisfied, the depth must be optimized with respect to the wavelength. However, since this is a very narrow bandwidth such as described above, the wavelength dependency of the diffraction efficiency can be substantially disregarded.
If, in equation (2), it is assumed that Nxe2x86x92∞, it follows that:
xcex7∞1=1.
It is seen therefrom that the diffraction efficiency is 100% in an idealistic case. However, from the diffraction efficiency and the smallest linewidth that can be produced, practically, a value about N=8 is used. The diffraction efficiency in that case is xcex781=0.95. Further, xcex78m takes a value only when m= . . . , xe2x88x9215, xe2x88x927, 1, 9, 17, . . . , such that it can be expressed as follows:
xcex78m=[sin(xcfx80m/8)/(xcfx80m/8)]2xe2x80x83xe2x80x83(3)
For example, xcex789=0.0117 and n87=0.0194 are obtained. Namely, for binary optics with eight steps, the diffraction efficiency becomes xe2x80x9cnot zeroxe2x80x9d at an order as represented by m=8k+1 (k is an integer). Generally, for steps N, the diffraction efficiency becomes xe2x80x9cnot zeroxe2x80x9d at an order represented by m=Nk+1 (k is an integer). Also, when the step-like shape is not formed idealistically, or when it is not made in conformity with the condition for application of the scalar diffraction theory, the diffraction efficiency at an order of an arbitrary integer m becomes xe2x80x9cnot zeroxe2x80x9d. In the following description, the light directed to orders other than the design order will be referred to as an xe2x80x9cunwanted diffraction orderxe2x80x9d. Particularly, the order Nk+1 (N is the number of steps and k is an integer other than zero), other than the design order, where the diffraction efficiency becomes xe2x80x9cnot zeroxe2x80x9d, under an idealistic condition (with an idealistic step shape), will be referred to as a xe2x80x9cmajor unwanted diffraction orderxe2x80x9d.
As described, when a binary optics element is used, there may be present unwanted diffraction orders appearing in particular order (directions). Since these light rays do not satisfy the imaging condition, when they are incident on an image plane, they appear as a flare component, causing degradation of the imaging characteristic. As regards the unwanted diffraction light which reaches the image plane, there are two types: light reflected once or more by a barrel, and light passing directly through the effective diameter of the optical system. As regards the light reflected by the barrel, it can be reduced sufficiently to a low level by designing the barrel and using anti-reflection. The light passing directly through the effective diameter of the optical system raises a problem.
When the influence of unwanted diffraction light is evaluated, both the intensity and the distribution of the unwanted diffraction light on the image plane with respect to the design order should be considered. The intensity should desirably be almost zero. However, as a tolerance, even if the intensity is about 1%, the distribution thereof on the image plane may be substantially uniform. On that occasion, since the unwanted diffracted light is added as an even (uniform) background light, the image contrast decreases slightly. Since, however, the contrast on the image plane as a whole is approximately even, it can be met by a subsequent process.
When a binary optics element is disposed adjacent to an object plane or image plane, the intensity of unwanted diffraction light on the image plane becomes larger and, additionally, exposure non-uniformness occurs. If it is disposed adjacent to a pupil plane of the projection optical system, the intensity is sufficiently small and substantially no exposure non-uniformness occurs. However, if the phase function applied to the binary optics element is sufficiently low, namely, the ring interval is sufficiently large, the intensity becomes large even though the element is disposed adjacent to the pupil plane. This can be explained qualitatively as that, as the ring interval becomes larger, the diffraction angle of (higher order) unwanted diffraction light becomes smaller such that the light passing through the effective diameter of the optical system increases. Therefore, in order to suppress the strength of the background light, the ring interval should be kept sufficiently small throughout the whole element. However, in a binary optics element having an ordinary condensing power, the ring intervals are very loose at the center while they are tight at the peripheral portion. It is, therefore, difficult to make the ring interval small over the whole element. This means that, as long as the number N of steps is fixed, unwanted light from a central region, where the ring interval is large, impinges on the image plane.
If the number N of the steps is made larger, the order of the unwanted diffraction light becomes largely different from the design order and thus the influence of the unwanted diffraction light is reduced. However, the size of N is limited by the production precision. For example, when an ordinary i-line stepper is used, the linewidth to be produced is about 0.3 micron. When a step-like structure with sixteen steps is made on the basis of it, the ring interval will be 5.6 microns. This value is not sufficient for completely correcting chromatic aberration in a projection optical system when a KrF excimer laser is used as a light source, and a narrow ring interval is required. Additionally, the manufacture of a step-like structure with sixteen steps requires the procedure including a plurality of processes. If plural processes are executed with a smallest linewidth close to the production limit, an intended step-like shape is not obtainable, and a production error easily occurs. Such a production error causes not only a major unwanted diffraction order m=Nk+1, but also lower order unwanted diffraction lights close to the design order.
It is accordingly an object of the present invention to provide an optical system with a diffractive optical element by which the influence of produced unwanted diffraction light to the imaging can be minimized, such that it can be applied as an optical system such as a projection optical system in a semiconductor manufacturing apparatus, for example, wherein an extraordinarily high imaging performance is required.
In accordance with an aspect of the present invention, there is provided a projection optical system with a diffractive optical element, characterized in that said diffractive optical element is arranged so that a portion of or most of diffraction light not to be used for projection of an image is prevented from being incident inside an image projection range upon an image plane.
In accordance with another aspect of the present invention, there is provided a projection optical system with binary optics, characterized in that the number of steps in each of the rings of said binary optics is determined in accordance with a ring interval of each ring so that a portion of or most of diffraction light not to be used for projection of an image is prevented from being incident inside an image projection range.
In this aspect of the present invention, said binary optics may be disposed adjacent to an aperture stop of said projection optical system.
When the number of steps is N and the ring interval is T, the following relation may be satisfied:
16xe2x89xa7Nxe2x89xa7T sin xcex8h/xcex,
where xcex is a representative value of a wavelength of light to be used with said projection optical system, and xcex8h is an angle defined, with respect to an optical axis of said projection optical system, by a light ray emitted from a largest object height of said projection optical system and passing through a center of said aperture stop.
Each ring of said binary optics may have at least eight steps.
In accordance with a further aspect of the present invention, there is provided a projection optical system with a diffractive optical element, characterized in that an aperture larger than an image to be projected on an image plane is provided adjacent to the image plane.
In accordance with a yet further aspect of the present invention, there is provided an optical system with a diffractive optical element, characterized in that a stop is provided adjacent to an image plane.
In these aspects of the present invention, an aperture stop may be provided at a position different from the position adjacent to the image plane.
A portion of diffraction light emitted from said diffractive optical element and not to be used for projection of an image may be intercepted by a light blocking portion of said stop.
Another portion of the diffraction light emitted from said diffractive optical element and not to be used for the image projection may be incident on an inside wall of a barrel of said optical system.
A further portion of the diffraction light not to be used for the image projection may pass through the aperture of said stop and may be superposed with the image, while a light intensity distribution of the further portion upon the image plane may be substantially uniform.
The diffractive optical element may comprise binary optics, and said optical system may include one or plural binary optics.
When the number of steps of each of the rings of said binary optics is N and the ring interval of each ring is T, the following relation may be satisfied:
16xe2x89xa7Nxe2x89xa7T sin xcex8h/xcex,
where xcex is a representative value of a wavelength of light to be used with said projection optical system, and xcex8h is an angle defined, with respect to an optical axis of said projection optical system, by a light ray emitted from a largest object height of said projection optical system and passing through a center of said aperture stop.
Each ring of said binary optics may have at least eight steps.
In accordance with a still further aspect of the present invention, there is provided a projection exposure apparatus for sequentially imaging a pattern of a mask on plural regions of a substrate to be exposed, by use of an optical system as recited above.
In accordance with a yet further aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: exposing a wafer with a device pattern by use of an exposure apparatus as recited above; and developing the exposed wafer.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.